It is known to separate the chlorides of potassium and sodium, which are the main components of sylvinite (sylvitic minerals), in a step in which the differential change in solubility of sodium chloride and potassium chloride in response to temperature changes is utilized. In the technologically interesting temperature range from about 20.degree. to 110.degree. C. the solubility of sodium chloride in the system potassium chloride/sodium chloride/water does not exhibit a significant change whereas the solubility of potassium chloride is more than doubled in response to an increase in temperature through this range.
For this reason it is conventional to leach sylvinite with reheated mother liquor recycled from a crystallizing stage so that potassium chloride is mainly dissolved and sodium chloride remains in the residue. The resulting liquor is then freed from other water-insoluble constituents, which are almost always present in sylvinite. The solution thus obtained is cooled to crystalline potassium chloride and is then reheated and recycled to the hot leaching step. (See Ullmann's Enzyklopadie der Techn. Chemie, 3rd edition, 1957, vol. 9, pages 185 et seq.).
In order to remove potassium chloride as completely as possible, a modification of the usual concurrent leaching process utilizes a main leaching stage, in which a major quantity of potassium chloride is dissolved in solvent liquor, and an after-leaching stage, in which remaining potassium chloride is dissolved also in fresh solvent liquor, and the last solution obtained is fed to the main leaching stage and is strengthened therein (see Ullmann's, loc.cit., page 189).
While the process outlined above is widely used, it has the disadvantage that a considerable amount of potassium chloride is lost in the water-insoluble sludges, to which a considerable amount of solution adheres, unless these losses are decreased by expensive washing steps.
Another disadvantage is that the thickening of the sludge and any washing thereof must be carried out at temperatures at which the liquors are highly corrosive so that special materials must be used in the equipment in which the treatment is carried out.